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DTC codes / B08xx · Airbags, seats, doors, lights and comfort systems / B0840

B0840Left Rear Turn Signal Circuit

B0840 indicates that the body control module (BCM) has detected a fault in the left rear turn signal circuit, which includes the wiring, bulb, and connections. The BCM monitors the circuit for abnormal voltage or current and sets this code when the expected electrical load is not present.

SystemBody
Code typeGeneric (all makes)
SeverityLow
Fix difficultyIntermediate
AreaAirbags, seats, doors, lights and comfort systems

Symptoms you may notice

  • Left rear turn signal does not illuminate when activated
  • Turn signal flashes rapidly (hyperflash) on the left side
  • Turn signal indicator on the dashboard may flash faster than normal
  • Often no symptoms besides the warning light if the fault is intermittent
  • Other turn signals may operate normally
  • Rear hazard flashers may also be affected on the left side

Common causes, most likely first

  1. Burnt out or faulty left rear turn signal bulb
  2. Corroded or damaged bulb socket
  3. Open or shorted wiring in the left rear turn signal circuit
  4. Poor ground connection at the left rear lamp assembly
  5. Faulty body control module (BCM) or turn signal switch
  6. Aftermarket LED bulb installation without proper load resistors
  7. Water intrusion or physical damage to the left rear lamp housing

How to diagnose and fix it

  1. Verify the left rear turn signal bulb is not burnt out and replace if necessary
  2. Inspect the bulb socket for corrosion, debris, or damage and clean or replace as needed
  3. Check the ground connection at the left rear lamp assembly for tightness and cleanliness
  4. Test the wiring harness for continuity and shorts between the BCM and the left rear lamp
  5. Inspect the turn signal switch for proper operation and replace if faulty
  6. Check for aftermarket LED bulbs and install load resistors or revert to stock bulbs
  7. If all else checks out, diagnose the BCM for internal faults

Common oversights when diagnosing B0840

  • A common mistake is replacing the BCM before checking the bulb socket, which often corrodes and causes an open circuit.
  • Aftermarket LED turn signal bulbs can trigger this code because their lower current draw mimics a burnt-out bulb to the BCM.
  • The left rear turn signal circuit shares a ground with other rear lamps, so a bad ground can cause multiple lighting issues.
  • Intermittent faults may only appear when the vehicle is driven over bumps, so wiggle-test the harness while monitoring live data.
  • Some vehicles use a smart junction box that requires reprogramming after replacement, which is often overlooked.

Can I keep driving with B0840?

It is generally safe to drive with this code as long as the turn signal is not completely inoperative, but you should avoid driving in conditions where signaling is critical until repaired. If the turn signal does not work at all, use hand signals and repair promptly to avoid a traffic citation.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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